Wireless communication system transmission methods have evolved to include multiple transmit antennas and multiple receive antennas in order to greatly increase the link capacity of wireless communication systems and/or to better focus the transmitted energy at the receiver for greater efficiency and less interference. An antenna array is a group of spaced apart antenna elements that each transmits an antenna signal that has a specific gain and phase relationship with the other antenna signals. When the antenna elements work together transmitting the antenna signals, they produce an antenna pattern that is more focused on the receiver than a pattern produced by a single antenna element. The process of changing the gain and phase of a signal to produce antenna signals may be referred to as “weighting” the signal using a set of “antenna array weights.” Because antenna arrays may similarly be used at a receiver to improve signal quality, use of antenna arrays at both the transmitter and receiver has been proposed. When multiple antenna elements are used at each of the transmitter and receiver, the wireless channel between them may be referred to as a multiple-input, multiple-output (MIMO) channel. Determining how to feed signals to the multiple transmit antenna elements and receive signals from the multiple receive antenna elements becomes quite complicated.
Various transmission strategies require the transmit antenna array to have some level of knowledge concerning the channel response between each transmit antenna element and each receive antenna element and are often referred to as “closed-loop” MIMO. Obtaining full broadband channel knowledge at the transmitter is possible using techniques such as uplink sounding in Time Division Duplexing (TDD) systems and channel feedback in either TDD or Frequency Division Duplexing (FDD) systems. Limited feedback methods, such as fixed beam selection or codebook-based beamforming weight selection, can reduce the amount of feedback as opposed to full channel feedback, which would require a significant amount channel resources thereby reducing the link capacity.
Codebook-based beamforming weight selection involves selection, by a base station based on feedback from a mobile station, of a set of pre-coding matrices, that is, predetermined beamforming weights that are agreed upon between a transmitter and receiver. The weights are selected from a set, or codebook, of predetermined and agreed upon matrices of beamforming weights. Each matrix in the codebook can be identified by an index, and a mobile station identifies the weights to be applied by feeding back an index to a matrix and rank of the matrix. In this way, only an index and a rank need be used in feedback to the transmitter in order for the transmitter to know the proper weights to use.
In order to provide such feedback, a base station conveys a midamble, that is, a predetermined signal, to a mobile station (MS). The midamble is not weighted, that is, no beamforming weights are applied to the midamble prior to transmission to the MS. Based on the received midamble, the MS computes a channel response for the air interface between the mobile station and the base station and, based on the channel response, determines a matrix and rank of weights for application to downlink transmissions. However, existing codebooks are quite complex, with weights for each index comprising a matrix of a rank of up to the number of antenna elements (the rank means the number of data streams that are to be transmitted). Selecting, by an MS, a matrix and rank for application to downlink transmissions requires a number of involved calculations, which can include complex math including matrix multiplications. For example, for codebook feedback by an MS, computations scale by at least a factor MtN as the number of antenna elements increases, where Mt is the number of transmit antenna elements at a base station and N is a number of indices in the codebook for each rank. For fixed beam selection, use of a precoded midamble reduces computational complexity but requires an increased midamble size, and correspondingly reduced downlink throughput, as compared to codebook techniques. That is, as a number of transmit antenna elements increases, midamble overhead scales as N for beam selection but as Mt for codebook feedback. For example, for four transmit antenna elements Mt=4 and N=16, and for eight transmit antennas elements Mt=8 and N=64.
Therefore, there is a need for a method and apparatus of closed loop feedback that reduces the computational complexity of the searching required for determining a codebook index and rank to feedback to the base station and that will not increase downlink overhead over the current art.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.